There is a virus sweeping the world. It infects opponents of the Bush government, sucks their brains out through their eyes and turns them into gibbering idiots. First cultivated in a laboratory in the US, the strain reached these shores a few months ago. In the past fortnight, it has become an epidemic. Scarcely a day now passes without someone possessed by this sickness, eyes rolling, lips flecked with foam, trying to infect me.

The disease is called Loose Change. It is a film made by three young men that airs most of the standard conspiracy theories about the attacks of September 11 2001. Unlike the other 9/11 conspiracy films, Loose Change is sharp and swift, with a thumping soundtrack, slick graphics and a calm and authoritative voiceover. Its makers claim that it has now been watched by 100 million people.

The Pentagon, the film maintains, was not hit by a commercial airliner. There was "no discernible trace" of a plane found in the wreckage, and the entrance and exit holes in the building were far too small. It was hit by a cruise missile. The twin towers were brought down by means of "a carefully planned controlled demolition". You can see the small puffs of smoke caused by explosives just below the cascading sections. All other hypotheses are implausible: the fire was not hot enough to melt steel and the towers fell too quickly. Building 7 was destroyed by the same means a few hours later.

Flight 93 did not crash, but was redirected to Cleveland airport, where the passengers were taken into a Nasa building and never seen again. Their voices had been cloned by the Los Alamos laboratories and used to make fake calls to their relatives. The footage of Osama bin Laden, claiming responsibility for the attacks, was faked. The US government carried out this great crime for four reasons: to help Larry Silverstein, who leased the towers, to collect his insurance money; to assist insider traders betting on falling airline stocks; to steal the gold in the basement; and to grant George Bush new executive powers, so that he could carry out his plans for world domination.

Even if you have seen or read no other accounts of 9/11, and your brain has not yet been liquidised, a few problems must occur to you. The first is the complete absence of scientific advice. At one point, the presenter asks: "So what brought down the twin towers? Let's ask the experts." But they don't ask the experts. The film-makers take some old quotes, edit them to remove any contradictions, then denounce all subsequent retractions as further evidence of conspiracy.

The only people they interview are a janitor, a group of firemen, and a flight instructor. They let the janitor speak at length, but cut the firemen off in mid-sentence. The flight instructor speaks in short clips, which give the impression that his pupil, the hijacker Hani Hanjour, was incapable of hitting the Pentagon. Elsewhere he has said the opposite: he had "no doubt" that Hanjour could have done it.

Where are the structural engineers, the materials scientists, the specialists in ballistics, explosives or fire? The film-makers now say that the third edition of the film will be fact-checked by an expert, but he turns out to be "a theology professor". They don't name him, but I would bet that it's David Ray Griffin, who also happens to be the high priest of the 9/11 conspiracists.

The next evident flaw is that the plot they propose must have involved tens of thousands of people. It could not have been executed without the help of demolition experts, the security firms guarding the World Trade Centre, Mayor Giuliani (who hastily disposed of the remains), much of the US air force, the Federal Aviation Administration and the North American Aerospace Defence Command, the relatives of the people "killed" in the plane crashes, the rest of the Pentagon's staff, the Los Alamos laboratories, the FBI, the CIA, and the investigators who picked through the rubble.

If there is one universal American characteristic, it is a confessional culture that permits no one with a good story to keep his mouth shut. People appear on the Jerry Springer Show to admit to carnal relations with their tractors. Yet none of the participants in this monumental crime has sought to blow the whistle - before, during or after the attacks. No one has volunteered to tell the greatest story ever told.

Read some conflicting accounts, and Loose Change's case crumbles faster than the twin towers. Hundreds of people saw a plane hit the Pentagon. Because it collided with one of the world's best-defended buildings at full speed, the plane was pulverised - even so, plane parts and body parts were in fact recovered. The wings and tail disintegrated when they hit the wall, which is why the holes weren't bigger.

The failure of the twin towers has been exhaustively documented by the National Institute of Standards and Technology. Far from being impossible, the collapse turns out to have been inevitable. The planes cut some of the support columns and ignited fires sufficient to weaken (but not melt) the remaining steel structures. As the perimeter columns buckled, the weight of the collapsing top stories generated a momentum the rest of the building could not arrest. Puffs of smoke were blown out of the structure by compression as the building fell.

Counterpunch, the radical leftwing magazine, commissioned its own expert - an aerospace and mechanical engineer - to test the official findings. He shows that the institute must have been right. He also demonstrates how Building 7 collapsed. Burning debris falling from the twin towers ruptured the oil pipes feeding its emergency generators. The reduction in pressure triggered the automatic pumping system, which poured thousands of gallons of diesel on to the fire. The support trusses weakened and buckled, and the building imploded. Popular Mechanics magazine polled 300 experts and came to the same conclusions.

So the critics - even Counterpunch - are labelled co-conspirators, and the plot expands until it comes to involve a substantial part of the world's population. There is no reasoning with this madness. People believe Loose Change because it proposes a closed world: comprehensible, controllable, small. Despite the great evil that runs it, it is more companionable than the chaos that really governs our lives, a world without destination or purpose. This neat story draws campaigners away from real issues - global warming, the Iraq war, nuclear weapons, privatisation, inequality - while permanently wrecking their credibility. Bush did capitalise on the attacks, and he did follow a pre-existing agenda, spelt out, as Loose Change says, by the Project for the New American Century. But by drowning this truth in an ocean of nonsense, the conspiracists ensure that it can never again be taken seriously.

The film's greatest flaw is this: the men who made it are still alive. If the US government is running an all-knowing, all-encompassing conspiracy, why did it not snuff them out long ago? There is only one possible explanation. They are in fact agents of the Bush regime, employed to distract people from its real abuses of power. This, if you are inclined to believe such stories, is surely a more plausible theory than the one proposed in Loose Change.